The present invention relates to tissue ablation and more particularly to methods and devices for creating effective ablation extending entirely through a layer of tissue. It has particular application to methods and devices for ablating foci and for forming conduction blocking lesions in a wall of the heart.
Over the last decade, the field of electrophysiology, especially mapping and treatment of arrhythmias of the heart, has grown spectacularly. A great number of ablation tools and catheters have been devised, and procedures have been methodically tested employing a number of catheter-like ablation devices, often with structures specifically constructed to enable the instrument to place a specific lesion at a site or region. Among catheter-based cardiac interventional techniques, the most common employ either high frequency (RF) electrical energy or cryogenic cooling to ablate tissue and cause scarring. Other available catheters employ laser illumination, microwave energy or heated surface contact to ablate tissue at a region or along a path operative to destroy aberrant tissue or disconnect an aberrant conduction pathway. These cardiac treatment methods require mapping of the aberrant foci within the heart, and then selective ablation of foci, or lines that isolate the foci, in the heart wall. Such ablation treatment is generally a time-consuming and complex procedure.
A number of these catheters are configured for percutaneous insertion to their point of application, for example, along a vessel into a chamber of the heart to form an endocardial lesion. Still others, both hand-manipulated and catheter-manipulated devices, may be configured for epicardial application. In addition, a hybrid approach has been developed which involves tools and devices similar to those developed for minimally invasive or endoscopic surgeries; these surgical or treatment devices reach the target tissue thoracoscopically through a small port or opening port passing through the myocardial wall to access the interior of the heart.
Each of these approaches has its own particular advantages and constraints. Thus, for example, basket catheter constructions are intended for delivery (typically along an endovascular route) into a cardiac chamber, and seek to provide an expanded electrode structure that may maintain itself in fixed contact with the endocardial wall of a still-beating heart for protracted time periods, so that mapping and ablation operations may be carried out in a fixed frame of reference at plural discrete points or along discrete arcs or segments. Still other catheters may have ablation tips with a particular geometry effective to provide oriented arc or segment lesions. Many common catheter devices act as drag electrodes with an ablation tip that xe2x80x9cdrawsxe2x80x9d lines of conduction block, ablating tissue at its point of contact as the tip is moved along the heart wall, allowing greater flexibility in placement, but at the expense of stability of positioning. Such catheter tips may require (and include) one or more barbs or prongs that penetrate the cardiac wall to anchor the ablation tip assembly.
Typically, the locations of lesions determines their effectiveness against particular re-entrant signal paths, so a preliminary mapping step is usually necessary. The requirement that a physician map sites or conduction patterns and then create a number of separate lesions positioned to destroy those sites or block re-entrant pathways, dictates that a cardiac treatment procedure be relatively lengthy. Often, once a lesion is created in a chamber wall, the chamber must be remapped to assure that block has occurred and that the tissue is not simply stunned. Thus it may be necessary to confirm conduction patterns after waiting a number of minutes, and it may also be necessary to re-ablate a lesion if the initial treatment lesion did not extend deeply enough to be effective, or was inaccurately placed or otherwise ineffective.
One area of particular interest is that of atrial fibrillation. Atrial fibrillation is a commonly occurring disorder characterized by erratic beating of the atrium, a condition that may result in thrombogenesis and stroke. While medication can be effective for some cases, many patients are not responsive to medical therapies, and effective treatment of those resistant cases calls for creating lesions in the atria to form effective conduction block. The Cox surgical MAZE procedure addresses this problem by stopping the patient""s heart, opening the atrium and dividing the chamber wall into pieces which are then sewn back together. The result of such surgery is to create scar tissue extending entirely through the cardiac wall and located along the cut lines. The scar tissue effectively blocks electrical conduction across the cut lines, and these divide the atrium into a number of sub-regions that are each too small to support a re-entry pathway. The surgical MAZE technique, however, requires that the heart be stopped, so it suffers from the morbidities associated with placing the patient on cardiopulmonary bypass and stopping the heart. It is also time-consuming and technically difficult to perform.
It has been found that a number of atrial arrhythmias originate at positions within a pulmonary vein and propagate into the atrial wall. Several specialized catheters have been described in the literature to treat these arrhythmias, with a construction that can be inserted through a heart chamber to a pulmonary vein and used to ablate a circumferential blocking lesion in the tissue of the vein or around the pulmonary os. U.S. Pat. No. 6,012,457 shows one such device. However, ablating circumferentially within a pulmonary vein can cause pulmonary vein stenosis. U.S. Pat. No. 6,161,543 also addresses the treatment of atrial fibrillation arising from such venous arhythmia sites, describing an ablation method and tool that percutaneously access the heart and penetrate the cardiac wall with an ablation tool to endocardially contact the atrial wall and form a cryogenic lesion surrounding the pulmonary veins.
Positive results have been reported for such systems. However, it also seems likely that a more complicated lesion set will be required to be fully effective in the majority of patients, and the variable thickness of the cardiac wall at the os may be quite variable, making the formation of a fully transmural blocking lesion problematic.
In addition to cardiac ablation catheters, a number of tools exist for ablating other tissue and performing common surgical tasks, such as coagulating vessels encountered during surgery to prevent bleeding. Many of these tools have a basic shape similar to a scalpel, forceps or other hand tool, with ablation electrodes positioned in a tip or jaw region. RF ablation tools of this sort may be monopolar or bipolar.
Monopolar ablation systems generally have a single electroded surface as the active ablating contact element, and utilize a large area return electrode to complete the current circuit. The return electrode is generally placed on the patient""s skin. Effective operation of such systems relies on the fact that current density becomes high, and ablation occurs, only in a small region close to where the active electrode contacts tissue. Bipolar systems, on the other hand, employ two closely spaced electrodes of opposite polarity to define the current paths through tissue. The current is high because the electrodes are near to each other, and both electrodes are of comparable size, so the high current region spans the tissue volume lying between the electrodes. Bipolar electrodes may be expected to form better-defined ablation lesions, since current will not vary unexpectedly as movement of the active electrode results in changing impedance pathways to the return electrode. However, the provision of a high intensity local current path between two electrodes poses the problem of positioning the tool without damaging unintended sites.
In the area of cardiac ablation, these problems are compounded since the cardiac chambers are normally full of blood, which is highly conductive and is prone to coagulation when overheated.
It is therefore desirable to provide ablation devices, systems and methods that dependably form a defined lesion extending entirely through a layer of tissue, such as cardiac tissue.
The present invention provides a method and a tool implementing the method, for safe and effective ablation of tissue. An ablation head is arranged to engage a surface of the target tissue, and two or more electrodes are arranged in a bipolar electrode configuration thereon to ablate the tissue. The electrodes are positioned with respect to the tissue engaged by the ablation head such that the inter-electrode current paths span the thickness dimension of the tissue layer. The method and device assure that ablation occurs through the full thickness of the tissue layer, while generally preventing the current paths from extending to adjacent volumes at which blood coagulation or unwanted tissue damage could occur.
The invention is quite general in scope. As applied to the heart, the heart may be stopped or beating, and the device may be configured so electrodes make endocardial or epicardial contact. Access to the target tissue may be either through an open incision, or by a minimally invasive technique involving a small opening through which one or more elongated surgical tools or endoscopic devices are inserted. As applied to cardiac tissue, methods and devices may, for example, form blocking lesions effective to prevent atrial fibrillation. Furthermore, a procedure in accordance with the present invention can be performed as either a stand-alone procedure, or may be carried out as a prophylactic procedure performed after a coronary artery bypass graft (CABG) operation to prevent perioperative onset of arrythmia, such as atrial fibrillation.
An ablation tool of the present invention has a handle at a proximal end, an ablation head at a distal end, and an elongated body interconnecting the handle and the ablation head. The ablation head includes a channel or other shape, and preferably also includes a structure or mechanism that grips or otherwise fixes the position of tissue that it contacts. A bipolar arrangement of electrodes defines localized tissue ablation along short paths between electrodes and through tissue secured by the ablation head structure. The channel and/or gripping mechanism immobilizes tissue in a position such that the conduction paths between the opposed electrodes span the thickness dimension of the tissue layer (e.g., the cardiac wall, in this example). The ablation head may grip with a suction force and tissue may be immobilized, for example, by means of suction apertures communicating with a vacuum passage, thus drawing the tissue down toward the apertures and bending the wall to effect a transmural ablation. A tissue-gripping force may alternatively be provided mechanically, by implementing the ablation tool as a pair of forceps or tongs with opposed electrodes formed on the jaws of the tool. Another embodiment has an ablation head having a contact face configured as a channel or as a concave wall, as seen in cross-section across a long axis. Two or more electrodes are positioned on the contact face to define short inter-electrode paths through the channel or concavity such that manually-exerted pressure of the ablation head against tissue forces the tissue into the channel or concavity and bends the tissue into position for through-layer ablation.
The electrodes and other ablation head structure may be rigidly affixed to the end of the elongated body, and may for example be affixed transversely to allow the body to push the contact face against tissue when manipulated by the handle, forming a through-wall ablation along the band of tissue in the channel. Alternatively, the ablation head may extend from the tool body such that drawing the handle along the axis of the body drags the electroded face along a path on the tissue surface, forming a through-wall ablation along a band of tissue in the path.
For treatment of a cardiac arrhythmia due to re-entrant pathways or signals originating in the region of the pulmonary veins or elsewhere, the ablation devices of the present invention allow effective bi-polar formation of lesions by contact with a single surface (endocardial or epicardial) of the cardiac wall while assuring that the legions extend entirely through the wall. The channel positions the tissue such that the primary conduction paths between electrodes of opposite polarity extend directly through the full layer of tissue. The transmural ablation energy paths so defined reduce the possibility of inducing coagulation in any blood present in the cardiac chamber.
Ablation heads for devices of the present invention may have relatively short or small electrodes for forming spot-like or drag-line ablation regions, or may have longer arc- or linear segment-electrodes for forming defined arcuate or segmental lesions. One ablation head is discretely sized to be manipulated between adjacent pulmonary veins for creating relatively small lesions in the cardiac wall, extending through the wall and circumferentially surrounding one or more of the veins, e.g., about one vessel, or about a pair of pulmonary veins. Another ablation head may be sized to function as a drag line ablator for forming lesions along a path, e.g., extending from a circumferential lesion at the pulmonary veins down to the mitral valve.
For use on the cardiac wall, an ablation device preferably has a bipolar head with opposed electrodes spaced approximately ten millimeters apart, across a channel which may have a depth, e.g., of about five millimeters. The channel is effective to allow cardiac wall tissue to enter the channel and position at least one transmural thickness directly in the current pathways between the opposed electrodes while substantially excluding the interior chamber (when applied epicardially) from crossing the current paths. The channel may be formed as a U-shaped channel with bipolar electrodes of opposed polarity on opposite sides thereof to form a single current path or band. Alternatively, the channel may have another shapexe2x80x94rectangular, circular, elliptical or simply a shallow dished concavity. Furthermore, additional electrodes may be provided such that ablation regions are defined by multiple path segments. Thus, for example, an electrode of one polarity may be located in the bottom of the channel and act together with several electrodes of the opposite polarity situated on respective side walls of the channel. Other embodiments have a curved or dished channel with bipolar electrode segments positioned to create a transmural lesion forming path when tissue is held therein. Preferably the channel operates as a vacuum chuck, with suction applied through internal passages maintained at a vacuum pressure of about 100-500 mm Hg to draw tissue into the channel and position it relative to the ablation electrodes. Most preferably, a seal extends along the contact face.